Magnetic thin film magnetic recording medium

ABSTRACT

A ferromagnetic thin film is used to provide a magnetic recording medium whose noise is low and whose S/N is high. The ferromagnetic thin film as the magnetic film of the magnetic recording medium is such that the fluctuation field of magnetic viscosity at 25° C. at the field strength equal to remanence coercivity or coercivity is not less than 15 oersteds and the coercivity is not less than 2000 oersteds.

BACKGROUND OF THE INVENTION

The present invention relates to a magnetic recording medium using aferromagnetic metal thin film, and more particularly to a magneticrecording medium having excellent electromagnetic transducingproperties, and a large capacity magnetic recording and reproducingapparatus.

For improving the recording density, increasing the output and reducingthe noise of magnetic recording media, it is essential to micronizemagnetic particles in the case of a coated medium and crystal grains inthe case of a thin film medium. Regarding a medium using metal particlesthat has heretofore been studied, for example, micronization hasprogressed and high-performance tapes such as Hi-8 (8-mm high-densitymagnetic tapes) using extra-fine particles having a cylinder major axislength of approximately 200 nm and a cylinder diameter of approximately30 nm are now put to practical use. Incidentally, a plurality ofparticles are subjected to magnetic reversal in a group and signals arerecorded when magnetic particles have been formed into a clusteragglomerate or when the interaction between crystal grains is strongeven though the magnetic particles or crystal grains of a magneticmedium are extremely fine. When the plurality of particles are subjectedto magnetic reversal and when the magnetic reversal unit becomes larger,noise increases at the time of reproducing data. In consequence, thedensity improvement is greatly hampered.

The size of the magnetic reversal unit is relevant to magneticviscosity. In other words, it is considered that the greater thefluctuation field of magnetic viscosity becomes, the smaller themagnetic reversal unit is. A description has been given of a meaning ofthe fluctuation field of magnetic viscosity in the Journal of Physics F:Metal Physics, Vol. 14, pp L155 to L159 (1984). Further, a detaileddescription has also been given of the measurement conditions in theJournal of Magnetism and Magnetic Materials, Vol. 127, pp 233 to 240(1993). The principle of measuring the fluctuation field of magneticviscosity will subsequently be described.

When a new magnetic field is applied to a magnetic material, themagnetization I(t) often varies in relation to the logarithm lnt of thefield applied time:

I(t)=const.+S·lnt.  (1)

In this case, I(t) represents a magnetic moment per unit volume; and t,elapsed time after the new magnetic field is applied. The viscositycoefficients has a positive value when the magnetic field is shifted inthe positive direction and has a negative value when it is shifted inthe negative direction. Moreover, it is known that S can be expressed bythe product of the irreversible susceptibility χ_(irr) and thefluctuation field H_(f). In other words, there is established.

S=χ_(irr)·H_(f)  (2)

Therefore, the fluctuation field is determined if S and χ_(irr) arefound experimentally. The fluctuation field is a quantity representingthe degree of the influence of thermal fluctuation, and a greaterfluctuation field signifies that it is easily affected by thermalfluctuation and that the magnetic reversal unit is small in size.

The fluctuation field where the field strength is equal to coercivity orremanence coercivity can also be found from the dependence on the fieldapplied time of the coercivity H_(c), or remanence coercivity H_(r), Thecoercivity or remanence coercivity, together with field applied time t,often lowers in relation to

H_(c)(or H_(r))=−A·lnt+const.  (3)

as the application time elapses. All the specimens mentioned in thepresent specification, satisfied the equation (3). When the coercivityor remanence coercivity varies with the field applied time t accordingto Eq. (3), it is known that A takes substantially the same value asthat of the fluctuation field H_(f) where the field strength is equal tothe coercivity or remanence coercivity. This procedure is not onlysimple but also excellent in reproducibility. Hence, the value A istaken as the fluctuation field of magnetic viscosity according to thepresent invention.

By measurement at room temperature, the fluctuation field thus found hasthe nature of becoming large in proportion to the absolute temperatureat the time of measurement. When a fluctuation field is measured at roomtemperatures ranging from 10° C. to 30° C. excluding 25° C. (theabsolute temperature: T) according to the present invention, thefluctuation field thus measured is multiplied by (298/T) to take theproduction as a fluctuation field H_(f) at 25° C.

In accordance with the conventional method, a Cr under-layer was firstformed on a mirror-polished disk made of Ni—P electroless-plated Al—Mgalloy, and then a CoCrTa magnetic layer together with a protectivecarbon film was formed thereon to fabricate a magnetic disk. The Crunder-layer, the magnetic layer and the protective layer were formed byAr-gas sputtering. In this case, the substrate temperature and the Arpressure were 300° C. and 2.0 milliTorr, respectively. Further, the Crunder-layer, the magnetic layer and the protective layer were 50 nm, 25nm and 10 nm thick, respectively. The composition of the CoCrTa magneticlayer is Co: 80%, Cr: 16%; Ta: 4%, expressed by atomic %. Thiscomposition will be expressed as CoCr₁₆Ta₄. The coercivity H_(c) and theremanence coercivity H_(r) were 1645 and 1655 oersteds, respectively.Further, the fluctuation fields of magnetic viscosity at 25° C. at thefield strength equal to the coercivity and at the field strength equalto the remanence coercivity were 13.5 and 13.2 oersteds, respectively.Thus the fluctuation fields of magnetic viscosity at 25° C. at the fieldstrength equal to the coercivity and at the field strength equal to theremanence coercivity exhibit substantially the same value: hereinafterthese are called simply the fluctuation field in this specification.Incidentally, the measuring time of the fluctuation field ranged from 0to 30 minutes.

A parmalloy head having a gap length of 0.4 μm and a coil of 24 turnswas used to record magnetic data on the medium, and a magneto-resistiveparmalloy head was used to reproduce the data in order to examine theelectromagnetic transducing properties. The flying height at the time ofrecording and reproducing data was 80 nm then. As a result ofmeasurement, noise at a longitudinal bit density of 150 kFCI (kilo FluxChange per Inch) was 22 μVrms.

Although a magnetic disk unit having a recording density of 300megabits/square inch could be fabricated by using this medium, amagnetic disk unit having a recording density of 1-gigabit/square inchcould not be fabricated.

An object of the present invention is to provide a magnetic recordingmedium and a magnetic recording and reproducing apparatus suitable forreducing noise at the time of reproducing data and for high-densityrecording.

SUMMARY OF THE INVENTION

FIG. 1 is an enlarged sectional view of a magnetic recording mediumembodying the present invention. In FIG. 1, reference numeral 1 denotesa nonmagnetic substrate of Ni—P-clad aluminum, Ni—P-cladaluminum-magnesium alloy, glass carbon or the like; 2, a nonmagneticunder-layer for controlling the crystal orientation and crystal grainsize of a magnetic film, which is a metallic layer of Cr, Cr—Mo, Cr—W,Cr—Ti, Cr—V or the like; 3, a ferromagnetic thin film of a cobalt-basedalloy such as Co—Cr—Ta, Co—Cr—Pt, Co—O, Co—Ni, Co—Cr, Co—Mo, Co—Ta,Co—Ni—Cr, Co—Ni—O or the like alloy; and 4, a protective lubricant layerin which a carbon film, an oxide film, a plasma polymerized film, fattyacid, perfluorocarbon carboxylic acid, perfluoropolyether or the likemay be used as a single or composite material. A ferromagnetic thin filmfor use as the magnetic layer 3 is desirably such that the fluctuationfield of magnetic viscosity at 25° C. at the field strength equal to theremanence coercivity or coercivity is not less than 15 oersteds, thecoercivity is not less than 2000 oersteds, and the thickness of themagnetic layer 3 is not less than 5 nm and not more than 30 nm. It ismore desirable that the fluctuation field of magnetic viscosity at 25°C. at the field strength equal to the remanence coercivity or coercivityis not less than 20 oersteds. The ferromagnetic thin film is desirably acobalt-based ferromagnetic thin film containing at least one kindselected from a group consisting of Cr, Ta, Pt, Ni, Mo, V, Ti, Zr, Hf,Si, W and O, for example, a thin film containing cobalt of Co—Cr—Ta,Co—Cr—Pt, Co—O, Co—Ni, Co—Cr, Co—Mo, Co—Ta, Co—Ni—Cr, Co—Ni—O or thelike.

A specific method for measuring the fluctuation field is as follows:

In order to obtain a fluctuation field A, a magnetic field of −10,000oersteds is applied to a specimen 7 mm square cut out of a magnetic diskbefore being subjected to dc-erase. Subsequently, a positive magneticfield slightly lower than the coercivity or remanence coercivity isapplied to the specimen to obtain time t until the magnetization orremanent magnetization decreases to zero. While the positive magneticfield applied after the dc-erase is lowered gradually, the operationabove is repeated. The fluctuation field A is found from the dependenceof the coercivity or remanence coercivity on the field applied time thusdetermined according to Eq. (3). The fluctuation field found from thedependence of the coercivity on the field applied time showssubstantially the same value as that of the fluctuation field found fromthe dependence of the remanence coercivity on the field applied time.Because of measurement simplicity, the fluctuation field A was foundfrom the dependence of the remanence coercivity on the field appliedtime according to the present invention. A vibrating sample magnetometerof DMS (Digital Measurement Systems) Co. was employed for themeasurement purposes. The measuring temperature was at 25° C. and thefield applied time after the dc-erase was in a range of 0 to 30 minutesthen.

In a region of a short time less than several seconds, data from 8seconds up to 30 minutes was used when the fluctuation field was foundsince an error in the applied time tends to become greater.

Although the magnetic disk was an object in the example above, thepresent invention is also effective for magnetic recording media such asmagnetic tapes.

When a ferromagnetic thin film whose fluctuation field of magneticviscosity at 25° C. at the field strength equal to the remanencecoercivity or coercivity is not less than 15 oersteds and whosecoercivity is not less than 2000 oersteds is used, and a magnetic layer3 whose thickness is not more than 5 nm and not less than 30 nm is used,it is possible to lower the noise level and to raise the S/N since thecluster size can be decreased at the time of magnetic reversal.

By the combination with a magnetic head using a metal magnetic film inpart of the magnetic pole, the medium capable of fast recording isallowed to demonstrate its performance, so that a large-capacityrecording and reproducing apparatus can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a magnetic recording and reproducingapparatus embodying the present invention.

FIG. 2 is a characteristic diagram showing the relations between thefluctuation field and the coercivity and between the fluctuation fieldand the noise.

FIG. 3 is a characteristic diagram showing the relation between thefluctuation field and the coercivity and between the fluctuation fieldand the noise.

FIG. 4 is a characteristic diagram showing the relation between thefluctuation field and the coercivity and between the fluctuation fieldand the

FIGS. 5(a) and 5(b) are sectional structural views of a magnetic diskunit.

DETAILED DESCRIPTION OF THE PREPARED EMBODIMENTS

Referring to an embodiment of the present invention, a detaileddescription will subsequently be given of the contents thereof.

[Embodiment 1]

A Cr-alloy under-layer was first formed on a mirror-polished disk madeof Ni—P electroless-plated Al—Mg alloy, and then a CoCrTa magnetic layertogether with a protective carbon film was formed thereon to fabricate amagnetic disk.

The Cr-alloy under-layer, the magnetic layer and the protective layerwere formed by Ar-gas sputtering. In this case, the Ar pressure was 2.0milliTorr. Cr—V, Cr—W, Cr—Ti, Cr—Si and Cr—Mo were used for Cr-alloyunder-layers to prepare 20 specimens in total different in under-layercomposition. The Cr-alloy layer, the magnetic layer and the protectivelayer were 50 nm, 25 nm and 10 nm thick, respectively. The compositionof the CoCrTa magnetic layer thus utilized was CoCr₁₆Ta₄. The substratetemperature at the time of forming the Cr-alloy under-layer and theprotective carbon film was 300° c., whereas the substrate temperature atthe time of forming the magnetic layer was 250-300° C.

The coercivities H_(c) of the media thus prepared were distributed in arange of 1500-2400 oersteds. The fluctuation fields were distributed ina range of 11.3-16.5 oersteds.

A parmalloy head having a gap length of 0.4 μm and a coil of 24 turnswas used to record magnetic data on the media, and a magneto-resistiveparmalloy head was used to reproduce the data in order to examine theelectromagnetic transducing properties. The flying height at the time ofrecording and reproducing data was 80 nm then. As a result ofmeasurement, the noise values at the longitudinal bit density of 150kFCI ranged from 18 to 25 μVrms. Table 1 collectively shows themeasurement results.

TABLE 1 Composition Thickness of magnetic of magnetic FluctuationCoercivity Noise film film (nm) field (Oe) (Oe) (μVrms) CoCr₁₆Ta₄ 2511.3 1500 25.0 CoCr₁₆Ta₄ 25 11.5 1601 24.6 CoCr₁₅Ta₄ 25 11.8 1685 24.3CoCr₁₅Ta₄ 25 12.1 1723 24.5 CoCr₁₅Ta₄ 25 12.3 1756 23.6 CoCr₁₆Ta₄ 2512.6 1832 23.2 CoCr₁₆Ta₄ 25 12.9 1889 22.6 CoCr₁₆Ta₄ 25 13.0 1890 22.5CoCr₁₆Ta₄ 25 13.1 1926 22.0 CoCr₁₆Ta₄ 25 13.2 1956 22.1 CoCr₁₆Ta₄ 2513.4 1985 21.8 CoCr₁₆Ta₄ 25 13.6 1989 21.5 CoCr₁₆Ta₄ 25 13.9 2023 21.3CoCr₁₆Ta₄ 25 14.1 2056 21.4 CoCr₁₆Ta₄ 25 14.3 2122 20.7 CoCr₁₆Ta₄ 2514.6 2146 20.2 CoCr₁₆Ta₄ 25 14.7 2250 20.5 CoCr₁₆Ta₄ 25 15.0 2280 19.5CoCr₁₆Ta₄ 25 15.5 2420 19.0 CoCr₁₆Ta₄ 25 16.5 2400 18.0

FIG. 2 shows the relations between the fluctuation field and thecoercivity and between the fluctuation field and the noise. As isobvious from FIG. 2, the noise values of the media whose fluctuationfields are of great values are low. The S/N values of the media ofhaving fluctuation fields of not less than 15 oersteds are higher thanthose of conventional media. It is therefore possible to make therecording density higher than conventional. The use of media withfluctuation fields of not less than 15.0 oersteds makes it possible tomanufacture magnetic disk units having a recording density of1-gigabit/square inch.

[Embodiment 2]

As in the first embodiment of the present invention, a Cr under-layerwas first formed on a mirror-polished disk made of a Ni—Pelectroless-plated Al—Mg alloy, and then a CoCrPt magnetic layertogether with a protective carbon film was formed thereon to prepare amagnetic disk.

The Cr under-layer, the magnetic layer and the protective layer wereformed by Ar-gas sputtering. In this case, the Ar pressures was 2.0milliTorr. By varying the Cr content of the CoCrPt magnetic layer, 20specimens in total having compositions ranging from CoCr₁₅Pt₈ toCoCr₂₃Pt₈ were fabricated. The Cr under-layer, the magnetic layer andthe protective layer were 50 nm, 25 nm and 10 nm thick, respectively.The substrate temperature at the time of forming the Cr under-layer, themagnetic layer and the protective carbon film was 300° C. The coercivityH_(c), of the media thus fabricated were distributed in a range of1800-2800 oersteds. The fluctuation fields were distributed in a rangeof 12.0-20.5 oersteds.

As in the first embodiment of the present invention, the electromagnetictransducing properties were measured. As a result the noise values atthe longitudinal bit density of 150 kFCI ranged from 17.9 to 30 μVrms.Table 2 collectively shows the measurement results.

TABLE 2 Composition Thickness of magnetic of magnetic FluctuationCoercivity Noise film film (nm) field (Oe) (Oe) (μVrms) CoCr₁₅Pt₈ 2512.0 1800 30.0 CoCr₁₅Pt₈ 25 12.6 1890 24.3 CoCr₁₆Pt₈ 25 12.2 1820 29.8CoCr₁₆Pt₈ 25 12.9 1850 26.3 CoCr₁₇Pt₈ 25 13.1 1920 23.6 CoCr₁₇Pt₈ 2513.3 2010 23.2 CoCr₁₇Pt₈ 25 13.7 1860 22.6 CoCr₁₈Pt₈ 25 13.3 2011 23.3CoCr₁₈Pt₈ 25 14.5 2306 22.1 CoCr₁₉Pt₈ 25 14.1 2215 22.2 CoCr₁₉Pt₈ 2514.6 2526 21.8 CoCr₂₀Pt₈ 25 14.9 2756 21.6 CoCr₂₀Pt₈ 25 15.0 2654 19.5CoCr₂₁Pt₈ 25 15.1 2345 18.8 CoCr₂₁Pt₈ 25 16.2 2645 18.8 CoCr₂₂Pt₈ 2515.3 2689 19.3 CoCr₂₂Pt₈ 25 16.8 2608 19.2 CoCr₂₃Pt₈ 25 17.5 2720 18.3CoCr₂₃Pt₈ 25 18.8 2800 19.1 CoCr₂₃Pt₈ 25 20.5 2750 17.9

FIG. 3 shows the relations between the fluctuation field and thecoercivity and between the fluctuation field and the noise. As isobvious form FIG. 3, the noise values of the media whose fluctuationfields have great values are conversely low as in the first embodimentof the present invention. The S/N value of media having fluctuationfields of not less than 15 oersteds are higher than those ofconventional ones. The use of media having fluctuation fields of notless than 15.0 oersteds enabled the manufacture of magnetic disk unitshaving a recording density of 1-gigabit/square inch. Moreover, the useof media having fluctuation fields of 20.5 oersteds and coercivity of2750 oersteds also enabled the manufacture of magnetic disk units havinga recording density of 1.5-gigabits/square inch. However, any one of themedia illustrated in this embodiment was unsuitable for producingmagnetic disks having a recording density of 2-gigabits/square inch.

[Embodiment 3]

A Cr under-layer was first formed on a mirror-polished glass disk, andthen a CoCrPt magnetic layer together with a protective carbon film wasformed thereon to prepare a magnetic disk.

The Cr under-layer, the magnetic layer and the protective layer wereformed by Ar-gas sputtering. In this case, the Ar pressure was 2.0milliTorr, and the composition of the CoCrPt magnetic layer utilized wasCoCr₁₉Pt₈. Then 30 specimens were fabricated by varying the thickness ofthe Cr under-layer from 3 up to 50 nm, varying the thickness of themagnetic layers from 3 up to 30 nm and setting those of the protectivelayer to 10 nm. The substrate temperature at the time of forming the Crunder-layer, the magnetic layer and the protective carbon film was 300°C.

The coercivities H_(c) of the media thus fabricated were distributed ina range of 1200-2900 oersteds. The fluctuation fields were distributedin a range of 11.2-68.3 oersteds.

As in the first embodiment of the present invention, the electromagnetictransducing properties were measured. As a result, the noise values atthe longitudinal bit density of 150 kFCI widely ranged from 8 to 31μVrms. Table 3 collectively shows the measurement results.

TABLE 3 Composition Thickness of magnetic of magnetic FluctuationCoercivity Noise film film (nm) field (Oe) (Oe) (μVrms) CoCr₁₉Pt₈ 3011.2 1200 31.0 CoCr₁₉Pt₈ 30 11.7 1321 29.6 CoCr₁₉Pt₈ 30 15.1 2140 19.6COCr₁₉Pt₈ 30 15.5 2206 21.9 CoCr₁₉Pt₈ 30 15.6 2518 21.9 CoCr₁₉Pt₈ 2712.1 1880 23.1 CoCr₁₉Pt₈ 27 13.2 1625 24.0 CoCr₁₉Pt₈ 27 13.8 1979 22.8CoCr₁₉Pt₈ 27 14.2 1959 21.5 CoCr₁₉Pt₈ 27 19.7 2356 18.2 CoCr₁₉Pt₈ 2512.4 1754 25.8 CoCr₁₉Pt₈ 25 15.3 1818 20.7 CoCr₁₉Pt₈ 25 16.5 2623 18.6CoCr₁₉Pt₈ 22 22.7 2218 18.8 CoCr₁₉Pt₈ 22 33.2 2756 16.7 CoCr₁₉Pt₈ 2023.6 2706 19.2 CoCr₁₉Pt₈ 20 25.8 2900 18.3 CoCr₁₉Pt₈ 15 26.4 2800 17.8CoCr₁₉Pt₈ 15 29.4 2300 17.6 CoCr₁₉Pt₈ 15 38.7 2356 15.6 CoCr₁₉Pt₈ 1221.5 1957 19.3 CoCr₁₉Pt₈ 10 39.6 2408 14.5 CoCr₁₉Pt₈ 10 44.3 2036 13.3CoCr₁₉Pt₈ 10 58.6 1789 14.2 CoCr₁₉Pt₈ 10 53.2 2013 11.3 CoCr₁₉Pt₈ 8 60.31802 10.3 CoCr₁₉Pt₈ 8 67.5 1400 9.5 CoCr₁₉Pt₈ 5 61.2 1830 8.9 CoCr₁₉Pt₈5 63.5 1750 8.3 CoCr₁₉Pt₈ 3 68.3 1540 8.0

FIG. 4 shows the relation between the fluctuation field and thecoercivity and between the fluctuation field and the noise. As isobvious from FIG. 4, the noise values of the media whose fluctuationfields have great values are conversely low as in the first and secondembodiments of the present invention. The use of media whose thicknessesof the magnetic films were 10-27 nm, whose fluctuation fields were notless than 15 oersteds and whose coercivities were not less than 2000oersteds permitted the manufacture of magnetic disk units having arecording density of 1-gigabit/square inch. Moreover, the use of mediawhose thicknesses of the magnetic films were 10-25 nm thick, whosefluctuation fields were not less than 20 oersteds and whose coercivitieswere not less than 2000 oersteds also permitted the manufacture ofmagnetic disk units having a recording density of 1.5-gigabits/squareinch. Further, the use of media whose thicknesses of magnetic films were10-22 nm, whose fluctuation fields were not less than 30 oersteds andwhose coercivities were not less than 2000 oersteds permitted themanufacture of magnetic disk units having a recording density of2-gigabits/square inch. In this embodiment, media whose coercivitieswere not less than 2000 oersteds could not be fabricated when thefluctuation fields exceeded 60 oersteds. The outputs of the media whosecoercivities were less than 2000 oersteds were low and besides eventhough the noise values were low, it was impossible to manufacturemagnetic disk units having a recording density of not less than1-gigabit/square inch. If, however, a medium having a coercivity of notless than 2000 or 3000 oersteds is produced even though the fluctuationfield exceeds 60 oersteds, a magnetic disk unit having a recordingdensity of 2-gigabits or greater may be manufactured. Notwithstanding,the influence of thermal fluctuation will become critical if thefluctuation field exceeds 1/20 of the coercivity, thus making the mediumpractically unusable. Although noise values were low in the case ofmedia whose magnetic films were less than 5 nm thick, sufficient outputswere not achieved, and consequently a magnetic disk unit having arecording density of not less than 1-gigabit/square inch could not beproduced using such media. If, further, the thickness of the magneticfilm exceeds 30 nm, demagnetization in recording due to the thick filmwas too great. As a result, no magnetic disk unit having a recordingdensity of 1-gigabit/square inch was produced.

[Embodiment 4]

FIG. 5 is a sectional structural view of a magnetic disk unitmanufactured by using media according to the present invention. In FIG.5, reference numeral 5 denotes magnetic recording media; 6, a magneticrecording medium drive; 7, a magnetic head; 8, a magnetic head drive;and 9, a recording and reproducing signal processor system. The use ofmagnetic recording media in the first to third embodiments of thepresent invention makes it possible to realize a recording density ofnot less than 1-gigabit/square inch.

As set forth above, according to the present invention, if aferromagnetic thin film whose fluctuation field of magnetic viscosity at25° C. at the field strength equal to the remanence coercivity orcoercivity is not less than 15 oersteds and whose coercivity thereof isnot less than 2000 oersteds is used, the S/N of the media can beremarkably improved, thus enabling high-density recording.

What is claimed is:
 1. A magnetic recording medium using a ferromagneticthin film as a magnetic layer whose fluctuation field of magneticviscosity at the field strength equal to remanence coercivity orcoercivity is not less than 15 oersteds.
 2. A magnetic recording mediumusing a ferromagnetic thin film as a magnetic layer whose fluctuationfield of magnetic viscosity at the field strength equal to remanencecoercivity or coercivity is not less than 20 oersteds.
 3. A magneticrecording medium using a ferromagnetic thin film as a magnetic layerwhose fluctuation field of magnetic viscosity at the field strengthequal to remanence coercivity or coercivity is not less than 30oersteds.
 4. A magnetic recording medium as claimed in one of the claims1, 2 and 3, wherein the coercivity of the ferromagnetic thin film is notless than 2000 oersteds.
 5. A magnetic recording medium as claimed inclaim 4, wherein the ferromagnetic thin film is a thin film of an alloymainly containing cobalt and selected from the group consisting ofCo—Cr—Ta, Co—Cr—Pt, Co—O, Co—Ni, Co—Cr, Co—Mo, Co—Ta, Co—Ni—Cr andCo—Ni—O.
 6. A magnetic disk using a magnetic recording medium as claimedin claim
 5. 7. A magnetic disk using a magnetic recording medium asclaimed in claim
 4. 8. A magnetic recording medium as claimed in one ofthe claims 1, 2 and 3, wherein the magnetic layer is not thinner than 5nm and not thicker than 30 nm.
 9. A magnetic recording medium as claimedin claim 8, wherein the ferromagnetic thin film is a thin film of analloy mainly containing cobalt and selected from the group consisting ofCo—Cr—Ta, Co—Cr—Pt, Co—O, Co—Ni, Co—Cr, Co—Mo, Co—Ta, Co—Ni—Cr andCo—Ni—O.
 10. A magnetic disk using a magnetic recording medium asclaimed in claim
 9. 11. A magnetic disk using a magnetic recordingmedium as claimed in claim
 8. 12. A magnetic recording medium as claimedin one of the claims 1, 2 and 3, wherein the ferromagnetic thin film isa thin film of an alloy mainly containing cobalt and selected from thegroup consisting of Co—Cr—Ta, Co—Cr—Pt, Co—O, Co—Ni, Co—Cr, Co—Mo,Co—Ta, Co—Ni—Cr and Co—Ni—O.
 13. A magnetic disk using a magneticrecording medium as claimed in claim
 12. 14. A magnetic disk using amagnetic recording medium as claimed in one of the claims 1, 2 and 3.15. A magnetic recording and reproducing apparatus for recording andreproducing data at a recording density of not less than1-gigabit/square inch by using a magnetic head with a ferromagnetic thinfilm as part of the magnetic pole, and the magnetic recording medium asclaimed in claim
 1. 16. A magnetic recording and reproducing apparatusfor recording and reproducing data with a recording density of not lessthan 1.5-gigabits/square inch by using a magnetic head with aferromagnetic thin film as part of the magnetic pole, and the magneticrecording medium as claimed in either claim 1 or
 2. 17. A magneticrecording and reproducing apparatus for recording and reproducing dataat a recording density of not less than 2-gigabits/square inch by usinga magnetic head with a ferromagnetic thin film as part of the magneticpole, and the magnetic recording medium as claimed in one of the claims1, 2 and 3.